Electrochemical corrosion pertains to destruction of the surface of a metal by electrochemical reaction. Such corrosion occurs if an electrolytic solution contacts the junction of two different metals. The corrosion element may be termed a short-circuited galvanic element.
On the element's anode, the less noble metal is oxidized. At the cathode, oxygen or hydrogen ions are reduced. Oxygen corrosion occurs in a neutral or alkali solution; while hydrogen corrosion is the result in solutions of higher pH or acidic solutions.
Electrochemical corrosion is a threat wherever two metallic conductors are in contact. Even foreign elements on a metallic surfaces may cause electrochemical corrosion. A film of water can be sufficient for electrolyte formation by exposure of metallic surfaces to the atmosphere. The rate of electrochemical corrosion is dependent upon conductivity of the electrolyte. Carbon dioxide absorption from ambient air can be partially transformed to carbonic acid in water, thereby contributing to electrochemical corrosion. Industrial emissions with sulphur dioxide, nitrogen oxides, acid gases, ammonia, amine and oxidizing gases and vapors increase the likelihood for metallic corrosion.
Protection from corrosion generally involves these considerations:
a) Positioning identical or electrochemically similar metals and alloys in a given milieu;
b) Preventing contact of electrolytic solutions with the junction between two different metals by application of protective coverings, coatings or metallic coverings; and
c) Cathodic protection can prevent corrosion of metals exposed to an electrolytic environment by electrically connecting the corrodible metal to a sacrificial anode made of a metal higher in the electromotive series than the metal for protection, i.e., a metal that is anodic to the material for protection. When the protected metal and the electrically connected sacrificial anode are both disposed within the same electrolytic environment, a galvanic cell is formed in which the protected material is the cathode, whereby metal atoms on the exposed surface of the sacrificial anode are ionized by the surrounding electrolyte and go into solution and the protected metal does not corrode since free electrons are readily available at the surface of that structure to chemically reduce or neutralize positive ions that reach the surface of the protected material.
Various measuring methods have been utilized in the field of protection against corrosion. Impedance spectroscopy, measurement of oxygen, hydrogen and pH value in solution, the redox potential, and weight control of metallic specimens inserted into the medium and determination of resistance.
Electrical resistance measurement is achieved with by insertion of pieces of wire, tubes or disks into the medium and exterior measurements indicating corrosion from the medium are taken. The change, diminution of size of the object, increases the resistance of the metallic specimen and, therefore, directly relates to the loss of metal by corrosion and/or erosion. The date can be converted to unit loss of metal per time unit to provide corrosion rate per year or similar time period.
Disadvantages concomitant with electrical resistance technique include the fact that the specimen itself is subjected to the signal current, for there is no galvanic separation between the metallic specimen and the medium to be measured. Furthermore, wire specimens are unstable and when used in a fluid moving at high rate, special protective devices for the metal specimen are required. Temperature compensation is unreliable due to disparity of the sacrificial and reference specimens. Changes in resistance of the specimens due to diminution of mass are small and range within milli and micro-Ohms. As such, recordation of signals is difficult and readily subject to extraneous influences. Also, the measurements can be erroneous in stronger electrolytes or by existence of electrical conducting depositions on the specimen.
Other modern sensors for corrosion measuring devices are "linear Polarization Probes", "Hydrogen Probes" and probes for impedance spectroscopy.
Recent reports from Russia indicate development of a method for magnetic measurements for the loss of mass due to corrosion of reinforcements in concrete bridge constructions by the use of "SQUID" gradiometers. Only initial tentative experiments have been reported.
The foregoing disclosures, however, have been found unsatisfactory in many respects. The disadvantage of these procedures is that they cannot be used on a technically large scale; they may be unreliable for sensitivity to extraneous influences; they may not be sufficiently durable and require repetitive calibration.